Page 11 - O Mahony Society Newsletter December 2024_Neat

Using the current O Mahony DNA Project data, I’ve used these criteria to define the data set:
• Only participants in haplogroup R were included

• Participants with data marked as private were excluded
• Names, including surnames and paternal ancestor names, have been removed; this carries the
assumption that every member of the O Mahony DNA Project claims or suspects descent from
a Mahony along their direct paternal ancestry
• Only participants with at least 67 STR markers were included

Since the O Mahony history tells that the most recent common ancestor of the O Mahonys is Donogh na
Himerce Tiomcuil O'Mahony, who lived about 1135 to 1212, we should narrow our search for haplogroups
belonging to the O Mahonys to branches that formed in the last thousand years or so. There are just three
groups among our participants who have branches with more than one member who reach that time period
with the formation of their defining SNP.

Haplogroup Members with confirmed SNP Members including clustered STR results

R-CTS4466 > R-S1121 25 56*
FGC5494 > R-BY155978 3 25
R-L266 10 15

*: clustering was not well defined for this group as described in the results

RESULTS

It is no surprise that the DNA results reveal several distinct branches of the O Mahonys with different lines
of descent. Even Rev. Canon O’Mahony knew this to be true in 1913, long before the structure of DNA had
even been discovered:
It would be unreasonable to suppose that the numerous families of the tribe, distinct from Mahon's,
that lived in 1035, had no descendants living in the seventeenth century. And, accordingly, it would seem
then that the hereditary surname does not imply that each one who bears it descends from the son of
Cian, which can be established only by proving descent from a chief or chief's relatives at the time of the
disruption of the sept.
Our modern genetic tools give us the unprecedented opportunity to discover which modern Mahonys
descend from which septs of the name. The results in this article are but one step along the path to our
understanding, and I’ll describe briefly here what new information this present research adds to the foundation
that was described in the 2017 paper.
R-CTS4466 > R-S1121: THE EÓGANACHTA BRANCHES

The R-CTS4466 haplogroup has long been associated with the Eóganachta clans of Munster, and the
R-S1121 subgroup is the largest one found among the Mahonys. That said, the genetic age of this haplogroup
predates the formation of the Eóganachta and has a distribution that extends beyond Ireland.
In 2021, Nigel McCarthy published Phylogenetic Alignments with Genealogies of Descent from Ailill Ólom,
which attempts to reconcile the historic genealogies of the Eóganachta descendants with the haplogroups
under R-CTS4466. That paper proposes that S-1121 > Z18170 represents the O Mahony descendants of
Áed Ualgarbh and S-1121 > A9005 represents the descendants of Laoghaire who would become our
Eóganachta cousins the O'Donoghues. He further proposes that the O’Reilly members of the FGC11140
child group might represent the descendants of Rahallagh, a paternal uncle of Mathghamhain. However,
for that explanation to hold up, we’d expect the FGC11140 SNP to have formed around 1000CE, but its
current estimated age is 400CE. Nigel acknowledged that there was a lack of DNA evidence for this group
at the time he did his research.
Unfortunately, I’ve reached the same conclusion in the present analysis. The research in 2017 predicted
that there were at least two groups of O Mahonys and that much has been supported by the more recent
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